Pneumatic holding device

ABSTRACT

A pneumatic holding device with at least one piston which can be impinged with a pressurized fluid medium at one face and displaceable in a cylinder against the action of a restoring force. The cylinder connection leading to the impinged face of the piston can be obturated by a valve mechanism which can be closed by the impinging fluid medium. The valve mechanism contains a valve body in which there is mounted a check or relief valve which opens in the direction of closing of the valve body. When open the valve mechanism communicates the impinged face of the piston with the face of the piston not impinged by the fluid medium.

Denzler et a1.

14 1 Dec. 10, 1974 Ott et a1 92/151 X Potepalov et a1 137/525 FORElGNPATENTS OR APPLICATIONS 11/1950 Sweden 91/447 Primary Examiner-Irwin C.Cohen Attorney, Agent, or Firm-Ostro1enk, Faber, Gerb &

ABSTRACT A pneumatic holding device with at least one piston which canbe impinged with a pressurized fluid medium at one face and displaceablein a cylinder against the action of a restoring force, The cylinderconnection leading to the impinged face of the piston can be obturatedby a valve mechanism which can be closed by the impinging fluid medium.The valve mechanism contains a valve body in which there is mounted acheck or relief valve which opens in the direction of closing of thevalve body. When open the valve mechanism communicates the impinged faceof the piston with the face of the piston not impinged by the fluid 2Claims, 2 Drawing Figures PNEUMATIC HOLDING DEVICE 3,485,141 12/19693,584,639 6/1971 [75] Inventors: Emil Denzler, Sprertenbach; RobertWidmer, Windisch, both of Switzerland 130 221 [73] Assignee: H. A.Schlatter A.G., Schlieren,

Switzerland [22] Filed: Mar. 29, 1972 soffen [21] Appl. No.: 239,143

[30] Foreign Application Priority Data Apr. 6, l97l Switzerland 5071/71[52] U.S. C1 91/440, 91/442, 137/493.1 [51] Int. Cl. ..F1Sb 11/08, Fl5b13/042 [58] Field of Search 91/440, 442, 268

[56] References Cited UNITED STATES PATENTS 1,441,759 1/1923 Schwennker91/442 1,681,579 8/1928 Farmer 92/78 2,888,909 6/1959 Gratzmu1ler....9l/442 3,307,454 3/1967 Larsson 92/161 x medlum- 3,456,561 7/l969Laikam, Jr 1. 91/440 3,464,321 9/1969 Piotrowski, Jr. 91/442 \I\ )KVX\IY \X 3 fi ze 9 "T a \LAL PNEUMATIC HOLDING DEVICE BACKGROUND TO THEINVENTION The present invention relates to a new and improvedconstruction of pneumatic holding or supporting device with at least onepiston which can be impinged by a pressurized fluid medium at one faceand which is displaceable in a cylinder against the action of arestoring or return spring.

Such devices are especially used in the construction of equipment where,during a given time, it is desired to generate a certain holding force,such as for instance for fixedly supporting a workpiece upon the worktable of a processing machine.

Among other things three requirements are placed upon such holdingdevices. Firstly, at the end of the work stroke the holding jawconnected with the piston rod should arrive at the workpiece as free aspossible from impact and only thereafter should there be generated thecomplete holding force. Secondly, the holding force, when no longerrequired, should be reduced as quickly as possible and the holding jawshould participate as quickly as possible in the return stroke. Thirdly,there should be present as little noise as possible, especially duringventing.

The first requirement has attempted to be previously fulfilled in thatthe build-up of pressure in the cylinder during the work stroke has beencontrolled, for instance, by supplying same via a throttle location orthrottle valve. Owing to the foregoing the second and third requirementscould not be previously simultaneously and optimally fulfilled. In theinterest of achieving a rapid return stroke the cylinder was eithersuddenly vented, while taking into account considerable noise, orattempts were made to reduce the development of noise by slowly ventingthe cylinder, but ,in such case there had to be taken into account thedrawback that pressure reduction only took place very slowly. I

SUMMARY OF THE INVENTION and is not associated with the aforementioneddrawbacks and limitations of the prior art proposals.

Another objective of the present invention relates to an improvedholding device or holding unit of the aforementioned type wherein allthree of the previously discussed requirements can be optimallyfulfilled.

Yet a further significant object of the present invention relates to anew and improved construction of pneumatic holding device which isrelatively simple in design, economical to manufacture, not readilysubject to breakdown, requires a minimum of maintenance and servicing,and enables the holding force to be both producedin a controlled mannerand quickly reduced, and affords relatively quiet operation of thesystem.

To implement these and still further objects of the invention, whichwill become more readily apparent as the description proceeds, theinvention is directed at designing the holding device in such a mannerthat the return force can be immediately effective independently of thepressure conditions prevailing at the cylin der, that is to say,independently of the momentary degree of venting, and as soon asthe'device is cut-off from the pressure source and vented.

In order to carry out the objectives of the invention the proposedholding device as contemplated by this development is manifested by thefeatures that the cylinder connection leading to the face or side of thepiston which is impinged by the pressurized fluid medium can be operatedby means of a valve mechanism which can be closed by the impinging fluidmedium. This valve mechanism has a valve body at which there is mounteda check or relief valve which opens in the closing direction of thevalve body. The valve mechanism in its open position communicates theimpinged face of the piston with the non-impinged face thereof.

Owing to the above-described construction as soon as the impinging fluidmedium is no longer present the valve mechanism is opened by thepressure still present in the cyliner. Consequently, there is obtainedan immediate pressure equalization between the impinged and thenon-impinged faces of the piston, enabling the restoring force to becomeimmediately effective.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be betterunderstood and objects other than those set forth above, will becomeapparent when consideration is given to the following detaileddescription thereof. Such description makes reference to the annexeddrawings wherein:

FIG. 1 is a schematic longitudinal sectional view through a preferredconstructional form of holding device or holding unit as designedaccording to the teachings of this invention; and

FIG. 2 is a fragmentary sectional view through a portion of a modifiedembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Describing now thedrawings, the exemplary illustrated embodiment of holding device orholding unit 1 depicted in FIG. 1 will be understood to comprise acylindrical housing 2, at one end or end face 3 of which there is formeda guide opening 5 equipped with a seal 4, this guide opening 5 guiding apiston rod 6 which extends towards the outside. A cylinder 8 is arrangedinternally of the housing 2 while maintaining free therebetween a jacketspace or outer compartment 7. In the embodiment under consideration thecylinder 8 possesses a so-called tandem construction, that is to say, itcontains two axially aligned cylinder compartments 9 and 10 separatedfrom one another by a partition of separation wall 11.

At the center of the partition wall 11 there is formed a substantiallyhub-shaped contact or impact support 12 which, in turn, possesses afurther guide bore 14 for the piston rod 6, guide bore 14 being equippedwith a seal 13, as shown.

The tandem construction of the cylinder 8 automatically leads to thesame type of construction for the piston 70. Specifically, in each ofthe cylinder compartments 9 and 10 there is mounted a piston plate 15and 16 respectively so as to be movable to-and-fro. As is standardpractice at the periphery of each piston plate 15 and 16 there isprovided a circumferential seal 17 by means of which, together with theassociated piston plate, each cylinder compartment 9 and 10 issubdivided into a chamber or section 9a and 10a respectively which isimpinged by pressurized fluid medium and a chamber or section 9b and 10brespectively which is not impinged by such fluid medium.

The piston plates and 16 are seated upon the common piston rod 6 inwhich between both piston plates 15 and 16, there is formed an axialbore 18 which at one end has ports or throughflow openings 19 leadingradially out of thepiston rod 6, as shown. Bore 18 and ports 19 thusalways flow communicate with one another the chambers 9a and 10a of thecylinder compartments 9 and 10 respectively which are impinged by thepressurized fluid medium.

Between the partition wall 11 and the non-impinged face 16a of thepiston plate 16 there is arranged a pressure or compression spring 20which in the pressureless condition of the system retains the piston 70in the position depicted in FIG. 1.

One end of the cylinder 8 is secured in any convenient manner to the endface 3 of the housing 2, whereas the cylinder 8 carries at its oppositeend a closure flange 21, at the center of which there is formed aconnection port or opening 22. The peripheral edge of this connectionopening 22 which protrudes past the closure flange 21 is constructed asa valve seat 23 for a valve plate 24 ofa valve mechanism or unit 71. Atthe flat face 24a of the valve plate 24 confronting the valve seat 23there is mounted a suitable sealing ring 25. Valve plate 24 also carriesa seal 29 at its periphery and, similar to the piston plates, is mountedto be displaceable longitudinally a valve compartment 27 formed at theother end face 26 of the housing 2. A connection opening or port 28leads to the valve compartment 27 and by means of which the illustratedholding device or unit 1 can be connected via a schematicallyillustrated compressed air conduit 31 and a suitable valve, such as theelectromagnetic 3/2-way valve 32 with a source of pressurized mediume.g. compressed air source 33.

Whereas the fluid medium-impinged chambers 9a and 10a of both cylindercompartments 9 and 10 respectively, as mentioned, are always in flowcommunication via the bore 18 and the throughflow openings or ports 19at the piston rod 6, the non-impinged chambers or sections 9b and 10b ofboth cylinder compartments 9 and 10 respectively, are likewisecontinuously in flow communication with one another. This isaccomplished in that there are provided at the walls of the cylindercompartment 9 and cylinder compartment 10, at the region of thedead-center points of the piston plates 15 and 16 respectively appearingat the left of the drawing, the openings or ports 34 and 35 whichconnect the associated cylinder compartments 9 and 10 directly with thejacket or outer compartment 7. Finally, the jacket compartment 7 iscontinuously connected via a small opening 36 with the ambient air.

The mode of operation of the illustrated embodiment of holder device orholder unit is as follows: If the valve 32 is moved out of itsillustrated position, then there is connected the compressed air source33. Thus initially the valve plate 24, under the action of the incomingcompressed air, is displaced towards the left in its valve closing orclosure position upon the valve seat 23. Consequently, the connection 22is cut-off from the jacket 4 or outer compartment 7. The check or reliefvalve opens and in accordance with its throughflow capacity compressedair flows to the impinged chambers 9a and 10a of the cylindercompartments 9 and 10 respectively. The thus occuring pressure build-up,after overcoming the force of the spring 20, displaces the piston plates15 and 16 towards the left until the piston plate 16 comes to bearagainst the free end face of the contact piece or stud 12. When thiscondition has been reached then the pressure in the fluid mediumimpingedchambers of the cylinder compartments 9 and 10 corresponds approximatelyto the pressure delivered by the compressed air source 33.

In this condition it would be possible to switch-off the compressed aitsource 33 by closing the conduit 31 or the throughflow opening 28. Ifthere is not taken into account possible leakage losses past the pistonseals 17 the holding unit will remain with the piston rod 6 in extendedposition because the valve mechanism 71 with the valve components23-24-25 remains closed. The reason for this resides in the fact that hesurface of the face 24b of the valve plate 24 confronting thecompartment 27 is greater than the surface of the other face 24a of thevalve plate 24 circumscribed by the ring seal 25.

Since both faces of the valve plate 24, that is to say, the face 24bconfronting the compartment 27 and the face 24a confronting theconnection 22 are impinged by the same pressure there is provided forthe valve plate 24 a resultant force effective towards the left of thedrawing.

Even with smaller leakage losses past the piston seals 17, i.e. with aslight pressure loss in the cylinder compartments, the check valve 30opens for pressure equalization between both faces of the valve plate24.

In the event that the return stroke should be initiated, then it issufficient to withdraw the valve 32 back into the illustrated solid lineposition. In so doing only the compartment 27, which possesses aconsiderably smaller volume than the stroke volume or swept capacity ofthe cylinder compartments 9 and 10, is vented. The check valve 30instantly closes and the valve plate 24 immediately shifts towards theright, so that the valve mechanism 71 with the valve components 23-2-4-25 is opened. As a result, however, there immediately takes place apressure equalization in the cylinder compartments 9 and 10 to bothsides or faces of the piston plates 15 and 16 owing to the jacketcompartment 7 and the openings 34 and 35, so that the spring 20immediately comes into play even if the total pressure prevailing in thehousing 2 is still considerably higher than theatmospheric pressure ofthe surroundings.

Independent of the return stroke of the pistons 15 and 16, with theopening of the plate valve unit 71 there begins the venting of theholder device 1 via the opening 36. Since the throughflow capacity ofthis opening 36 is however limited the quantity of air located in theholder unit is not suddenly expanded, which otherwise could lead to theheretofore known and experienced undesired noise. Quite to the contrary,venting of the device takes place slowly, which in the most favorablesituation is accompanied by a hissing sound which can be readilytolerated. Additionally, those skilled in the art will recognize thateven when venting of the device has not yet been completed it ispossible to initiate the next work stroke. In practice the volumecontent of the jacket compartment 7 with regard to the inner volume orswept capacity of the cylinder 2 and with regard to thethroughflow-capacity of the opening 36 are accommodated to one anotherin such a manner that presupposing a periodic work stroke the jacketcompartment 7 so to speak acts like an expansion chamber or surge tankand venting occurs continuously via the opening 36.

The illustrated and described construction of holder unit or holderdevice of this invention possesses a still further advantage. Since thereturn stroke of the piston 70 embodying the piston plates 15, 16 takesplace at a higher pressure than the atmospheric pressure of thesurroundings, during the return stroke there does not occur any suctionaction upon the non-impinged face of the piston plates 15, 16.Accordingly, measures are also provided which ensure that nocontaminants are sucked-up from the surroundings which, as is wellknown, could considerably reduce the longevity of the seals, in thiscase the seals 4, 13 and 17. i

It should be understood that the invention is not limited to theexemplary illustrated tandem construction. If it is imagined forinstance for the illustrated embodiment of holder device 1 that theopening 35 is closed, the bore 18 closed, there is omitted the pistonplate and the partition wall 11 is displaced to the left and coincideswith the end wall 3, then there is realized a construction of holderdevice which as far as its external dimensions are concerned isapproximately of the same size, however, has approximately twice thestoke with approximately half the force.

Continuing, in FIG. 2 of the drawings there is illustrated a variantembodiment of inventive holder device or unit in which the valvemechanism 71 embodying the components 23, 24, is indirectly controlledby the impinging fluid medium, i.e. is servo-controlled. By referringnow to FIG. 2 more specifically there will be recognizeda portion of thehousing 2 in which there is arranged the cylinder 8 while againmaintaining free therebetween the jacket or outer compartment 7. Theclosure flange 21 of the cylinder 8 is constructed in the same manner asfor the embodiment of FIG. 1. The valve plate 24 on the other hand isnot only mounted so as to be displaceable to-and-fro within the valvecompartment 27, rather additionally is subjected to the action ofcompression or pressure springs which strive to displace valve plate 24to the right of FIG. 2, that is into its open position. Furthermore, apiston extension or projection 41 is formed at the valve plate 24,which, in turn, is displaceably mounted in a piston compartment 42 andleads to the check or relief valve 30 via the onethroughflow opening 43communicating with such check valve. The piston projection 41 carries atone end a seal 44 which is destined to cooperate with the throughflowopening 46 at the piston compartment 42, which throughflow opening 46was a valve seat 45. The throughflow opening 46 opens into the inlet 47to the holder device which is directly connected with a compressed airnetwork.

A further throughflow opening 48 leads from this inlet 47 into the valvecompartment 49 of a solenoid valve 50. From this valve compartment 49there leads a further continuously open throughflow opening 51 into thevalve compartment 27 as well as past the plunger-type armature 52 of thesolenoid valve which is subjected to the action of a compression orpressure spring 53, to a throughflow opening or channel 54 which opensinto the jacket or outer compartment 7. The plunger-typearmature 52 isprovided at both' end faces with the seals SSwhich serve to close themouths of the throughflow openings 48 and 54 depending upon the positionof such armature 52.

The mode of operation of this embodiment of the invention is as follows:It is assumed that the inlet 47 is directly connected to a compressedair source and that the coil of the solenoid valve 50 is not excited.Accordingly, the plunger-type armature 52 is in its extended positionand closes by means of its seal 55 the mouth of the throughflow opening48. The throughflow openings 51 and 54 are in communication with oneanother and therefore also the face 24b of the valve plate 24 facingaway from the cylinder 8 with the face 24a of the valve plate 24confronting the cylinder 8. Thus at both sides of the valve plate 24there prevails the same pressure, causing the compression springs 40 toraise the valve plate 24 from the valve seat 23 and the therewithconnected piston 41 to shift until the throughflow opening 46 is closedby the seal 44. At this point it is to be mentioned that the compressionor pressure springs 40 are designed such that their force overcomes theforce of the pressure at the inlet 47 upon the surface circumscribed bythe seal 44 of the piston extension 41. If however the inlet 46 to thepiston compartment 42 is closed then no compressed air arrives at thethroughflow opening 43 and therefore at the cylinder 8. The holder unitremains in its retracted position.

Now if the coil of the solenoid valve 50 is excited or energized, thenthe plunger-type armature 52 is pulledin and therefore the throughflowopening 48 is freed, whereas on the other hand the throughflow opening54 is closed-off. Now compressed air flows via the throughflow opening48, the valve compartment 49 and the throughflow opening 51 into thevalve compartment 27 of the valve plate 24, in which compartmentpressure builds up, the action of which upon the much larger freesurface of the valve plate 24 (in comparison to the surface at thepiston extension 41 circumscribed by the seal 44) is capable ofovercoming the force of the compression or pressure springs 40. Hencethe valve plate 24 in FIG. 2 is thus displaced towards the left, that isto say, by means of the seal 25 is pressed onto the valve seat 23. Atthe same time, however, the piston extension 41 is lifted off of thevalve seat 45 and thus compressed air flows without obstruction throughthe throughflow opening 43 from the inlet 47. As a result thethrottlecheck valve 30 is opened and at the cylinder 8 there takes placethe build up of pressure which displaces the piston (as the pistons15,16 in FIG. 1) to the left has already been described in conjunctionwith the embodiment of FIG. 1. This condition is maintained for suchtime as the coil of the solenoid valve 50 is energized, that is, forsuch time as the through flow opening 48 is connected via thecompartment or chamber 49 with the throughflow opening 51.

As soon as however the coil of the solenoid 50 is deenergized then thethroughflow opening 48 is closed and the throughflow opening 54 isopened. Consequently, there occurs a pressure equalization between theface 24a of the valve plate 24 confronting the cylinder 8 and the face24b thereof which faces away therefrom. Consequently, the compressionsprings 40 again comes into play which then raise the valve plate 24away from the valve seat 23 and, on the other hand,

again close the inlet 46. Hence, as already described in conjunctionwith the embodiment of FIG. 1, there now however also takes place viathe jacket compartment 7 an immediate pressure equalization between bothfaces of the work piston of the holding devices which pistonthusimmediately undertakes its return stroke retracting into cylinder 21.

In contrast to the embodiment of FIG. 1, with this embodiment of FIG. 2for initiating the return stroke of the work piston there is not evendirectly vented to the surroundings the already relatively smallcompartment volume of the valve compartment 27, rather the return strokeof the valve plate 24 is initiated by bringing about a pressureequalization to both faces or sides. Furthermore, with the constructionas described in FIG. 2 only a very small control valve is required.Whereas for the embodiment of FIG. 1 the valve 32 must possess asufficiently great throughflow capacity in order to satisfy the take-upcapacity of the unit within an acceptable time, the solenoid valve 50 ofthe embodiment of FIG. 2 need only possess a nominal width of itsthroughflow of a few millimeters. Accordingly the solenoid valve 50 canbe selected to be so small that it can be integrated as a unit in theend wall 26 of the housing.

According to a further feature of this development the reduction innoise realized with both embodiments, and particularly that of FIG. 2can be even further increased. Instead of the aeration of vent opening36 which connects the jacket or outer compartment 7 functioning as theexpansion chamber with the external air, it is possible to form thecylindrical wall of the housing 2 entirely or partially from a limitedair permeable material, for instance by a felt supported by ribs, from aglass fiber structure or from a single or multiple layer wire mesh withvery narrow mesh size. This has been conveniently schematicallyindicated in FIG. 2 by reference character 2a. Consequently, all of thepressure surges which arise at the moment of pressure equalizationbetween both faces of the piston plates 15, 16 and the valve plate 24 atthe jacket or outer compartment 7 are not directly vented to thesurrounding air, rather only gradually as a function of the throughflowcapacity of the vent opening 36 and the limited throughflow capacity ofthe pervious housing wall 2a as above described.

The described holding device or holding unit of this developmenttherefore will be recognized to simultaneously fulfill in an idealmanner all of the previously mentioned three requirements.

While there is shown and described present preferred embodiments of theinvention, it is to be distinctly understood that the invention is notlimited thereto but may be otherwise variously embodied and practicedwithin the scope of the following claims. Accordingly,

What is claimed is:

l. A pneumatic holding device, comprising: a cylinder having a first andan opposite second end;

a partition intermediate the ends of said cylinder dividing saidcylinder into separate first and second piston compartments with saidfirst compartment being the closer one to said cylinder first end;

a piston rod for supporting and guiding pistons, said piston rodextending through both said piston compartments toward said cylinderfirst and second ends;

a first and a second piston supported upon said piston rod andlocated,respectively, in said first and said second piston compartments of saidcyliner; both said pistons being sealingly movable through therespective said compartments and each said piston thereby divides itsrespective said compartment; said pistons being spaced apart on said rodand being simultaneously movable by said rod; said pistons and saidpiston rod being shiftable axially through said cylinder toward saidcylinder ends; both said pistons having a respective first face facingtoward said cylinder first end and having a respective second facefacing toward said cylinder second end; means normally biasing saidpistons toward said cylinder first end;

pressurized fluid inlet having a first cross section and being connectedto said cylinder; a valve located between said pressurized fluid inletand said cylinder; said valve having an inlet communicating with saidfluid inlet and an outlet communicating with said cylinder; said valveoutlet communicating into said cylinder first compartment at a locationsuch that pressure may be applied through said fluid inlet to the saidfirst face of said first piston; a pressure transmitting conduitcommunicating between said valve outlet and said cylinder secondcompartment at a location such that pressure may be applied through saidfluid inlet to said first face of said second piston;

said valve comprising: a valve element compartment ofa second crosssection larger than said first cross section of said pressurized fluidinlet; a movable valve element in said valve element compartment, saidvalve element having a cross section such that it is sealably movablethrough said valve element compartment and being movable in said valveelement compartment toward and away from said cylinder under theinfluence of pressure in said valve element compartment and in saidcylinder; said valve element having a pressure inlet side facing towardsaid valve inlet and said fluid inlet and having a pressure outlet sidefacing toward said valve outlet and said cylinder;

narrowed passage through said valve element extending across said valveelement compartment to join said fluid inlet with said cylinder; aone-way check valve in said narrowed passage permitting only inlet tosaid cylinder through said narrowed passage;

said cylinder carrying a valve seat for said valve element against whichsaid valve element pressure outlet side is scalable; when said valveelement pressure outlet side, which said section has a thirdcross-section which is smaller than said valve element secondcross-section and which said valve element section is exposed to thesections of said first and said second compartments and thereby isexposed to the cylinder pressure at said first side of both saidpistons; with said valve element moved through its said compartment ontosaid valve seat, the entire said pressure inlet side of said valveelement is exposed to the inlet pressure in said fluid inlet and in saidvalve element compartment;

pressure equalization conduit connected to both said piston compartmentsin said cylinder at respective locations in said compartments such thatsaid pressure equalization conduit is always at the pressure at bothsaid piston second faces; said pressure equalization conduit alsocommunicating with said piston compartments at respective locations sothat it is in communication with said first sides of therefor;

means communicating said equalization conduit di- 10 compartment towardand away from said cylinder under the influence of pressure in saidvalve element compartment and in said cylinder; said valve elementhaving a pressure inlet side facing toward both said pistons; on thepath between the piston said fluid inlet and a pressure outlet sidefacing tocompartment sides facing said second piston sides ward saidcylinder; and the piston compartment sides facing said first a narrowedpassage through said valve element expiston sides, said equalizationconduit passes said tending across said valve element compartment tovalve element and said valve seat, such that with join said fluid inletwith said cyliner; a one-way said valve element on said valve seat, saidequalizacheck valve in said narrowed passage permitting tion conduit isblocked, and with said valve eleonly inlet to said cylinder through saidnarrowed ment off said valve seat, said equalization conduit passage; isopen; said cylinder carrying a valve seat for said valve elecylinderhousing surrounding and spaced from said ment against which said valveelement pressure cylinder and in which said cylinder is mounted andoutlet side is scalable; when said valve element is defining, betweensaid cylinder and said cylinder on said seat, said seat defines asection of said valve housing, said equalization conduit in the form ofa element pressure outlet side, which said section has jacketcompartment around the exterior of said a third cross-section which issmaller than said cylinder; valve element second cross-section and whichsaid said cylinder housing having opposed end faces; said valve elementsection is exposed to the cylinder valve means being in a valve housingformed at one pressure at said first face of said piston; with said endface of said cylinder housing surrounding said valve element movedthrough its said compartment cylinder; the other end face of saidcylinder housonto said valve seat, the entire said pressure inlet inghaving a guide opening for said piston rod and side of said valveelement is exposed to inlet pressaid piston rod passing through saidguide opening sure in said fluid inlet and in said valve elementcompartment;

pressure equalization conduit connected to said cylinder at a locationsuch that said pressure equalization conduit is always at the pressureat said piston second face; said pressure equalization conduit alsocommunicating with said cylinder at the side of said cylinder whichfaces said piston first 2. A pneumatic holding device, comprising: acylinder having a first and an opposite second end;

at least one piston in said cylinder and shiftable therethrough axiallythereof toward said cylinder ends; said piston having a first facefacing toward side and so communicating with said cylinder past saidvalve element and said valve seat such that said cylinder first end anda second face facing towith said valve element on said valve seat, saidward said cylinder second end; means normally biequalization conduit isblocked and with said valve asing said piston toward said cylinder firstend; element off said valve seat, said equalization cona pressurizedfluid inlet of a first cross section conduit is open;

nected to said cylinder at a location such that presmeans for ventingsaid equalization conduit for relief sure may be applied through saidfluid inlet to said 40 of pressure in that said conduit; saidequalization piston first face; conduit being in the form of ajacketcompartment avalve between said fluid inlet and said cylinder; said onthe exterior of said cylinder and said jacket valve comprising:compartment being enclosed by an outer wall; said a valve elementcompartment of a second crossventing means comprising at least a portionof said section larger than said first cross-section of said jacketcompartment outer wall being formed of a inlet; material having limitedair permeability, whereby a movable valve element in said valve elementcomsaid equalization conduit is gradually vented partment and having across-section such that it is through said outer wall portion of limitedair persealably movable through said valve element commeability.partment and being movable in said valve element

1. A pneumatic holding device, comprising: a cylinder having a first andan opposite second end; a partition intermediate the ends of saidcylinder dividing said cylinder into separate first and second pistoncompartments with said first compartment being the closer one to saidcylinder first end; a piston rod for supporting and guiding pistons,said piston rod extending through both said piston compartments towardsaid cylinder first and second ends; a first and a second pistonsupported upon said piston rod and located, respectively, in said firstand said second piston compartments of said cyliner; both said pistonsbeing sealingly movable through the respective said compartments andeach said piston thereby divides its respective said compartment; saidpistons being spaced apart on said rod and being simultaneously movableby said rod; said pistons and said piston rod being shiftable axiallythrough said cylinder toward said cylinder ends; both said pistonshaving a respective first face facing toward said cylinder first end andhaving a respective second face facing toward said cylinder second end;means normally biasing said pistons toward said cylinder first end; apressurized fluid inlet having a first cross section and being connectedto said cylinder; a valve located between said pressurized fluid inletand said cylinder; said valve having an inlet communicating with saidfluid inlet and an outlet communicating with said cylinder; said valveoutlet communicating into said cylinder first compartment at a locationsuch that pressure may be applied through said fluid inlet to the saidfirst face of said first piston; a pressure transmitting conduitcommunicating between said valve outlet and said cylinder secondcompartment at a location such that pressure may be applied through saidfluid inlet to said first face of said second piston; said valvecomprising: a valve element compartment of a second cross section largerthan said first cross section of said pressurized fluid inlet; a movablevalve element in said valve element compartment, said valve elementhaving a cross section such that it is sealably movable through saidvalve element compartment and being movable in said valve elementcompartment toward and away from said cylinder under the influence ofpressure in said valve element compartment and in said cylinder; saidvalve element having a pressure inlet side facing toward said valveinlet and said fluid inlet and having a pressure outlet side facingtoward said valve outlet and said cylinder; a narrowed passage throughsaid valve element extending across said valve element compartment tojoin said fluid inlet with said cylinder; a one-way check valve in saidnarrowed passage permitting only inlet to said cylinder through saidnarrowed passage; said cylinder carrying a valve seat for said valveelement against which said valve element pressure outlet side issealable; when said valve element pressure outlet side, which saidsection has a third cross-section which is smaller than said valveelement second cross-section anD which said valve element section isexposed to the sections of said first and said second compartments andthereby is exposed to the cylinder pressure at said first side of bothsaid pistons; with said valve element moved through its said compartmentonto said valve seat, the entire said pressure inlet side of said valveelement is exposed to the inlet pressure in said fluid inlet and in saidvalve element compartment; a pressure equalization conduit connected toboth said piston compartments in said cylinder at respective locationsin said compartments such that said pressure equalization conduit isalways at the pressure at both said piston second faces; said pressureequalization conduit also communicating with said piston compartments atrespective locations so that it is in communication with said firstsides of both said pistons; on the path between the piston compartmentsides facing said second piston sides and the piston compartment sidesfacing said first piston sides, said equalization conduit passes saidvalve element and said valve seat, such that with said valve element onsaid valve seat, said equalization conduit is blocked, and with saidvalve element off said valve seat, said equalization conduit is open; acylinder housing surrounding and spaced from said cylinder and in whichsaid cylinder is mounted and defining, between said cylinder and saidcylinder housing, said equalization conduit in the form of a jacketcompartment around the exterior of said cylinder; said cylinder housinghaving opposed end faces; said valve means being in a valve housingformed at one end face of said cylinder housing surrounding saidcylinder; the other end face of said cylinder housing having a guideopening for said piston rod and said piston rod passing through saidguide opening therefor; means communicating said equalization conduitdirectly with the ambient air for relief of pressure in that saidconduit.
 2. A pneumatic holding device, comprising: a cylinder having afirst and an opposite second end; at least one piston in said cylinderand shiftable therethrough axially thereof toward said cylinder ends;said piston having a first face facing toward said cylinder first endand a second face facing toward said cylinder second end; means normallybiasing said piston toward said cylinder first end; a pressurized fluidinlet of a first cross section connected to said cylinder at a locationsuch that pressure may be applied through said fluid inlet to saidpiston first face; a valve between said fluid inlet and said cylinder;said valve comprising: a valve element compartment of a secondcross-section larger than said first cross-section of said inlet; amovable valve element in said valve element compartment and having across-section such that it is sealably movable through said valveelement compartment and being movable in said valve element compartmenttoward and away from said cylinder under the influence of pressure insaid valve element compartment and in said cylinder; said valve elementhaving a pressure inlet side facing toward said fluid inlet and apressure outlet side facing toward said cylinder; a narrowed passagethrough said valve element extending across said valve elementcompartment to join said fluid inlet with said cyliner; a one-way checkvalve in said narrowed passage permitting only inlet to said cylinderthrough said narrowed passage; said cylinder carrying a valve seat forsaid valve element against which said valve element pressure outlet sideis sealable; when said valve element is on said seat, said seat definesa section of said valve element pressure outlet side, which said sectionhas a third cross-section which is smaller than said valve elementsecond cross-section and which said valve element section is exposed tothe cylinder pressure at said first face of said piston; with said valveelement moved through its said compartment onto said valve seat, theentire said pressure inlet Side of said valve element is exposed toinlet pressure in said fluid inlet and in said valve elementcompartment; a pressure equalization conduit connected to said cylinderat a location such that said pressure equalization conduit is always atthe pressure at said piston second face; said pressure equalizationconduit also communicating with said cylinder at the side of saidcylinder which faces said piston first side and so communicating withsaid cylinder past said valve element and said valve seat such that withsaid valve element on said valve seat, said equalization conduit isblocked and with said valve element off said valve seat, saidequalization conduit is open; means for venting said equalizationconduit for relief of pressure in that said conduit; said equalizationconduit being in the form of a jacket compartment on the exterior ofsaid cylinder and said jacket compartment being enclosed by an outerwall; said venting means comprising at least a portion of said jacketcompartment outer wall being formed of a material having limited airpermeability, whereby said equalization conduit is gradually ventedthrough said outer wall portion of limited air permeability.